GR with ref to outbound delivery

Hello,

For Intra co STO,  we are creating  replensihment delivery. On doing PGI, goods move to "stock in transit (movement type 641 ). GR is done with ref to outbound delivery in MIGO (movement type 101).

1) I need to know how the system knows that the GR for outbound delivery is pending and GR needs to be done (is soem flag set somewhere or some status is updated in STO ?)

2) Once GR is done, how the system knows that GR is done and the same outbound delivery should not be picked again for GR.

3) how can I do mass GR for outbound delivery for an STO ?

regards

Pamela